ltc2873 - single-bus rs485/rs232 multiprotocol …€¦ · loopback mode steers the driver inputs...

LTC2873

12873fa

For more information www.linear.com/LTC2873

Typical applicaTion

FeaTures DescripTion

Single-Bus RS485/RS232 Multiprotocol Transceiver with

Switchable Termination

The LTC®2873 is a robust pin-configurable multiprotocol transceiver that supports RS232, RS485, and RS422 protocols while operating on a single 3V to 5.5V supply. The LTC2873 can be configured as a half-duplex RS485 transceiver or as an RS232 transceiver using the same two bus pins.

A pin-controlled integrated termination resistor allows for easy interface reconfiguration, eliminating external resis-tors and control relays. Loopback mode steers the driver inputs to the receiver outputs for diagnostic self-test. The RS485 receiver supports up to 256 nodes per bus, and features full failsafe operation for floating, shorted or terminated inputs.

An integrated DC/DC boost converter uses a tiny 2mm × 1.6mm inductor and one capacitor, eliminating the need for multiple supplies when driving RS232 levels.applicaTions

n One RS485 or One RS232 Transceiver n 3V to 5.5V Supply Voltage n Up to 20Mbps RS485 n Slew-Controlled RS232 Operation:

n Selectable 1Mbps or 250kbps n Automatic Selection of Integrated RS485 (120Ω)

and RS232 (5kΩ) Termination Resistors n High ESD: ±26kV HBM n Logic Loopback Mode n 1.7V to 5.5V Logic Interface n Supports Up to 256 RS485 Nodes n RS485 Receiver Failsafe Eliminates UART Lockup n H-Grade Available (–40°C to 125°C) n Available in 24-Pin 4mm × 5mm QFN Package

n Software Selectable RS232/RS485/RS422 Interface n Industrial Sensors and Actuators n Alarm Systems n Traffic Control and Monitoring n Highway Signs and Jumbo Displays

L, LT, LTC, LTM, Linear Technology, the Linear logo and µModule are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.

2873 TA01

120Ω

5k1μF

1μF

1μF

1.7V TO VCC 3V TO 5.5V220nF 10μH

2.2μF

VL CAP SW

SHDN

DI

RO

485/232485/232

D/R(485)

RE485TE485DE485/F232LBGND

RS232

RS485

A/DO

B/RI

VDD

VEE

VCC

DATA IN

DATA OUT

LTC2873

RS232(485/232

LOW)

RS485(485/232

HIGH)

RS485/RS232 Mode Switching

RS232 RS485

1µs/DIV

DI

485/2325V/DIV

A/DO3V/DIV

B/RI

2873 TA01b

http://www.linear.com/LTC2873


LTC2873

22873fa


pin conFiguraTionabsoluTe MaxiMuM raTings

Input Supplies VCC, VL .................................................... –0.3V to 7VGenerated Supplies VDD ................................................ VCC – 0.3V to 7.5V VEE .........................................................–7.5V to 0.3VSW ............................................... –0.3V to (VDD + 0.3V)CAP ................................................. (VEE – 0.3V) to 0.3VA/DO, B/RI ...................................................–15V to 15VDI, 485/232, DE485/F232, RE485,

TE485, LB ................................................ –0.3V to 7VSHDN, RO .......................................–0.3V to (VL + 0.3V)Differential Terminator Voltage (Enabled) (A/DO to B/RI) ..................................................... ±6VDifferential Terminator Voltage (Disabled) (A/DO to B/RI) ................................................... ±30VOperating Temperature LTC2873C ............................................... 0°C to 70°C LTC2873I .............................................–40°C to 85°C LTC2873H .......................................... –40°C to 125°CStorage Temperature Range .................. –65°C to 150°C

(Notes 1 and 2)

8 9

TOP VIEW

UFD PACKAGE24-LEAD (4mm × 5mm) PLASTIC QFN

10 11 12

24 23 22 21 20

6

5

4

3

2

1VEE

RO

485/232

RE485

DE485/F232

DI

SHDN

NC

VCC

A/DO

GND

B/RI

VCC

VDD

TE48

5

LB V L V CC

GND

GND

CAP

V EE

GND

SW

7

14

15

16

17

18

19

13

25VEE

TJMAX = 150°C, θJA = 43°C/W

EXPOSED PAD (PIN 25) IS VEE, MUST BE SOLDERED TO PCB

orDer inForMaTionLEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE

LTC2873CUFD#PBF LTC2873CUFD#TRPBF 2873 24-Lead (4mm × 5mm) Plastic QFN 0°C to 70°C

LTC2873IUFD#PBF LTC2873IUFD#TRPBF 2873 24-Lead (4mm × 5mm) Plastic QFN –40°C to 85°C

LTC2873HUFD#PBF LTC2873HUFD#TRPBF 2873 24-Lead (4mm × 5mm) Plastic QFN –40°C to 125°C

Consult LTC Marketing for parts specified with wider operating temperature ranges. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/. Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix.

http://www.linear.com/product/LTC2873#orderinfo


http://www.linear.com/leadfree/

http://www.linear.com/tapeandreel/

http://www.linear.com/product/LTC2873#orderinfo

LTC2873

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The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VCC = VL = 3.3V, TE485 = VL, LB = 0V unless otherwise noted. (Notes 2, 6)

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

Supplies

VCC Supply Voltage Operating Range l 3 5.5 V

VL Logic Supply Voltage Operating Range VL ≤ VCC l 1.7 VCC V

VCC Supply Current in Shutdown Mode SHDN = 0V l 8 30 µA

VCC Supply Current in RS232 Mode or RS485 Mode, Driver and Receiver Enabled, Termination Disabled

No Load, SHDN = TE485 = DE485/F232 = VL RE485 = 0

l 4 9 mA

VCC Supply Current in RS485 Mode with Receiver and Termination Enabled, Driver Disabled

No Load, SHDN = 485/232 = VL DE485/F232 = RE485 = TE485 = 0

l 4 9 mA

VL Supply Current in Any Mode No Load l 0 5 µA

Power Supply Generator

VDD Regulated VDD Output Voltage SHDN = VL, No Load 7.0 V

VEE Regulated VEE Output Voltage SHDN = VL, No Load –6.3 V

RS485 Driver

|VOD| Differential Output Voltage RL = Open, VCC = 3V (Figure 1) RL = 27Ω, VCC = 4.5V (Figure 1) RL = 27Ω, VCC = 3V (Figure 1) RL = 50Ω, VCC = 3.13V (Figure 1)

l

l

l

l

2.1 1.5 2

VCC VCC VCC

V V V V

∆|VOD| Difference in Magnitude of Differential Output Voltage for Complementary Output States

RL = 27Ω, VCC = 3V (Figure 1) RL = 50Ω, VCC = 3.13V (Figure 1)

l

l

0.2 0.2

V

VOC Common Mode Output Voltage RL = 27Ω or 50Ω (Figure 1) l 3 V

∆|VOC| Difference in Magnitude of Common Mode Output Voltage for Complementary Output States

RL = 27Ω or 50Ω (Figure 1) l 0.2 V

IOSD485 Maximum Short-Circuit Current –7V ≤ VOUT ≤ 12V (Figure 2) l ±250 mA

RS485 Receiver

IIN485 Input Current (A/DO, B/RI) (A/DO or B/RI) = 12V or –7V, VCC = 0V or 3.3V (Figure 3)

l –100 125 µA

RIN485 Input Resistance (A/DO, B/RI) (A/DO or B/RI) = 12V or –7V, VCC = 0V or 3.3V (Figure 3)

125 kΩ

Differential Input Signal Threshold Voltage (A/DO to B/RI)

–7V ≤ (A/DO or B/RI) ≤ 12V l ±200 mV

Input Hysteresis B = 0V 220 mV

Differential Input Failsafe Rising Threshold Voltage –7V ≤ (A/DO or B/RI) ≤ 12V, (A/D0 – B/RI) Rising l –200 –70 –20 mV

Input DC Failsafe Hysteresis 40 mV

VOL Receiver Output Low Voltage Output Low, I(RO) = 3mA (Sinking), 3V ≤ VL ≤ 5.5V

l 0.4 V

Output Low, I(RO) = 1mA (Sinking), 1.7V ≤ VL < 3V

l 0.4 V

elecTrical characTerisTics


LTC2873

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VOH Receiver Output High Voltage Output High, I(RO) = –3mA (Sourcing), 3V ≤ VL ≤ 5.5V

l VL – 0.4 V

Output High, I(RO) = –1mA (Sourcing), 1.7V ≤ VL < 3V

l VL – 0.4 V

Three-State (High Impedance) Output Current (RO) 0V ≤ RO ≤ VL, VL = 5.5V, RE485 = VL l 0 ±5 µA

Short-Circuit Current (RO) 0V ≤ RO ≤ VL, VL = 5.5V l ±135 mA

RTERM Terminating Resistor TE485 = 0V, VAB = 2V, VB = –7V, 0V, 10V (Figure 8)

l 108 120 156 Ω

RS232 Driver

VOLD Output Low Voltage RL = 3kΩ, VEE ≤ –6V l –5 –5.5 VEE V

VOHD Output High Voltage RL = 3kΩ, VDD ≥ 6.5V l 5 5.9 VDD V

Output Short-Circuit Current Driver Output = 0V l ±25 ±90 mA

RS232 Receiver

Input Threshold Voltage l 0.6 1.5 2.5 V

Input Hysteresis l 0.1 0.4 1.0 V

Output Low Voltage I(RO) = 1mA (Sinking), 1.7V ≤ VL < 5.5V l 0.4 V

Output High Voltage I(RO) = –1mA (Sourcing), 1.7V ≤ VL < 5.5V l VL – 0.4 V

Input Resistance –15V ≤ B/RI ≤ 15V, 485/232 = 0V l 3 5 7 kΩ

Output Short-Circuit Current VL = 5.5V, 0V ≤ RO ≤ VL l ±25 ±50 mA

Logic Inputs

Threshold Voltage l 0.4 0.75 • VL V

Input Current l 0 ±5 µA

ESD

Interface Pins (A/DO, B/RI) Human Body Model to GND or VCC, Powered or Unpowered (Note 5)

±26 kV

All Other Pins Human Body Model (Note 5) ±4 kV

The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VCC = VL = 3.3V, TE485 = VL, LB = 0V unless otherwise noted. (Notes 2, 6)elecTrical characTerisTics


LTC2873

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RS485 Switching Characteristics

Maximum Data Rate (Note 3) (Figure 15) l 20 Mbps

tPLHD485, tPHLD485 Driver Propagation Delay RDIFF = 54Ω, CL = 100pF (Figure 4) l 20 70 ns

Driver Propagation Delay Difference RDIFF = 54Ω, CL = 100pF (Figure 4) tPLHD485 – tPHLD485

l

0

±6

ns

tSKEWD485 Driver Skew (A/DO to B/RI) RDIFF = 54Ω, CL = 100pF (Figure 4) 0 ±8 ns

tRD485, tFD485 Driver Rise or Fall Time RDIFF = 54Ω, CL = 100pF (Figure 4) 7.5 12.5 ns

tZLD485, tZHD485, tLZD485, tHZD485

Driver Output Enable or Disable Time SHDN = VL, RL = 500Ω, CL = 50pF, DE485 ↑ and ↓ (Figure 5)

l 120 ns

tZHSD485, tZLSD485 Driver Enable from Shutdown Time (Note 7)

DE485/F232 = VL, RL = 500Ω, CL = 50pF, SHDN ↑ (Figure 5)

l 4 12 µs

tHZSD485, tLZSD485 Driver Output Disable Into Shutdown Time

DE485/F232 = VL, RL = 500Ω, CL = 50pF, SHDN ↓ (Figure 5)

l 0.5 1 μs

tPLHR485, tPHLR485 Receiver Input to Output Time CL = 15pF, VCM = 1.5V, |A/DO to B/RI| = 1.5V, (Figure 6)

l 45 85 ns

tSKEWR485 Differential Receiver Skew tPLHR485 – tPHLR485

CL = 15pF (Figure 6) l 0 ±9 ns

tRR485, tFR485 Receiver Output Rise or Fall Time CL = 15pF (Figure 6) l 3 15 ns

tZLR485, tZHR485, tLZR485, tHZR485

Receiver Output Enable or Disable Time

485/232 = SHDN = VL, RL = 1kΩ, CL = 15pF, RE485 ↓ and ↑ (Figure 7)

l 12 85 ns

tZHSR485, tZLSR485 Receiver Enable from Shutdown Time (Note 7)

485/232 = VL , RE485 = 0V, RL = 1kΩ, CL = 15pF , SHDN ↑ (Figure 7)

l 4 12 µs

tHZSR485, tLZSR485 Receiver Output Disable Into Shutdown Time

485/232 = VL, RE485 = 0V, RL = 1kΩ, CL = 15pF ,SHDN ↓ (Figure 7)

l 0.5 1 µs

tRTEN485, tRTZ485 Termination Enable or Disable Time

485/232 = VL, SHDN = VL, B = 0, (A/DO to B/RI) = 2V (Figure 8)

l 100 µs

RS232 Switching Characteristics

Maximum Data Rate (Figure 15) RL = 3kΩ, CL = 2.5nF, (Fast, Slow Modes) RL = 3kΩ, CL = 1nF, (Fast, Slow Modes) RL = 3kΩ, CL = 0.25nF, (Fast Mode)

l

l

l

100 250

1000

kbps kbps kbps

Driver Slew Rate (Figure 9) RL = 3kΩ, CL = 2.5nF, (Fast, Slow Modes) RL = 3kΩ, CL = 50pF, (Slow Mode) RL = 3kΩ, CL = 50pF, (Fast Mode)

l

l

l

2 30

150

V/µs V/µs V/µs

tPHLD232, tPLHD232 Driver Propagation Delay (Figure 9) RL = 3kΩ, CL = 50pF, (Slow Mode) RL = 3kΩ, CL = 50pF, (Fast Mode)

l

l

1.5 0.4

3 1

µs µs

tSKEWD232 Driver Skew (Figure 9) RL = 3kΩ, CL = 50pF, (Slow Mode) RL = 3kΩ, CL = 50pF, (Fast Mode)

0 0

±400 ±100

ns ns

tZLSD232, tZHSD232 Driver Enable from Shutdown Time (Note 7)

VDD = 7.0V, VEE = –6.3V, 485/232 = 0V, RL = 3kΩ, CL = 50pF, SHDN ↑ (Figure 10)

l 5 12 µs

tLZSD232, tHZSD232 Driver Output Disable into Shutdown Time

485/232 = 0V, RL = 3kΩ, CL = 50pF, SHDN ↓ (Figure 10)

l 0.6 2 µs

tPHLR232, tPLHR232 Receiver Propagation Delay CL = 150pF (Figure 11) l 60 200 ns

tSKEWR232 Receiver Skew CL = 150pF (Figure 11) 25 ns

tRR232, tFR232 Receiver Output Rise or Fall Time CL = 150pF (Figure 11) l 70 200 ns

The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VCC = VL = 3.3V, TE485 = VL, LB = 0V unless otherwise noted. (Notes 2, 6)swiTching characTerisTics


LTC2873

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The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VCC = VL = 3.3V, TE485 = VL, LB = 0V unless otherwise noted. (Notes 2, 6)swiTching characTerisTics


tZLSR232, tZHSR232 Receiver Enable from Shutdown Time (Note 7)

VDD = 7.0V, VEE = –6.3V, 485/232 = 0V, RL = 1kΩ, CL = 150pF, SHDN ↑ (Figure 12)

l 5 12 µs

tLZSR232, tHZSR232 Receiver Disable Into Shutdown Time 485/232 = 0V, RL = 1kΩ, CL = 150pF SHDN ↓ (Figure 12)

l 0.4 2 μs

Mode Change Characteristics

tRDY VDD and VEE Supply Rise Time (Time from Shutdown to RS485 Ready)

(Figure 13) l 0.2 1 ms

tDR232 Time from RS485 Mode to RS232 Mode RS232 Driver Ready

(Figure 14) l 0.2 1 µs

tR232 Time from RS485 Mode to RS232 Mode RS232 Receiver Ready

(Figure 14) l 0.8 3 µs

tDR485 Time from RS232 Mode to RS485 Mode RS485 Driver and Receiver Ready

(Figure 14) l 70 250 ns

Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime.Note 2: All currents into device pins are positive; all currents out of device pins are negative. All voltages are referenced to device ground unless otherwise specified.Note 3: Guaranteed by other measured parameters and not tested directly.Note 4: Time from SHDN ↑ until VDD ≥ 5V and VEE ≤ –5V. External components as shown in the Typical Application section.

Note 5: Guaranteed by design and not subject to production test.Note 6:Testing was done with VDD and VEE back driven to valid supply levels for functions that require these supplies, unless otherwise noted.Note 7: If enabling from shutdown, where VDD and VEE supplies are collapsed, allow the extra time it takes to generate valid VDD and VEE supplies (tRDY).

VCC Supply Current vs Supply Voltage in Shutdown Mode

VCC Supply Current vs Supply Voltage in RDY Mode

VCC Supply Current vs RS485 Data Rate

Typical perForMance characTerisTics TA = 25°C, VCC = VL = 3.3V, unless otherwise noted.

SUPPLY VOLTAGE (V)3 3.5 4 4.5 5 5.5

5

6

7

8

9

10

11

12

13

14

15

SUPP

LY C

URRE

NT (μ

A)

2873 G01

SUPPLY VOLTAGE (V)3

SUPP

LY C

URRE

NT (m

A)

5

4

3

2

14.5 53.5

2873 G02

5.54

–40°C

25°C

85°C

DRIVER, RECEIVER AND TERMINATORDISABLED

DATA RATE (Mbps)0.1

SUPP

LY C

URRE

NT (m

A)

100

80

60

40

20

010

2873 G03

1001

TE485 LOW

TE485 HIGH

VCC = 5VVCC = 3.3V

RS485 DRIVER AND RECEIVERSWITCHING.CL = 100pF ON EACHDRIVER OUTPUT.


LTC2873

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VCC Supply Current vs RS232 Data Rate in Slow Mode

VCC Supply Current vs RS232 Data Rate in Fast Mode

RS485 Driver Differential Output Voltage vs Temperature

RS485 Driver Propagation Delay vs Temperature

RS485 Driver Skew vs Temperature

RS485 Driver Short-Circuit Current vs Short-Circuit Voltage

RS485 Receiver Propagation Delay vs Temperature

RS485 Receiver Skew vs Temperature

RS485 Receiver Output Voltage vs Load Current

2.5nF

2.5nF

1nF

1nF

75pF

75pF

VARIOUS LOADSA/DO TIED TO B/RIRECEIVER ENABLEDDRIVER AND VCC = 5V

VCC = 3.3V

DATA RATE (kbps)0 50 100 150 200 250

4

8

12

16

20

SUPP

LY C

URRE

NT (m

A)

2873 G04

2.5nF

2.5nF

1nF

1nF75pF

75pF

VARIOUS LOADSA/DO TIED TO B/RIRECEIVER ENABLED

250pF

250pF

DRIVER AND VCC = 5VVCC = 3.3V

DATA RATE (kbps)0 200 400 600 800 1000

4

8

12

16

20

24

SUPP

LY C

URRE

NT (m

A)

2873 G05TEMPERATURE (°C)

–50

VOLT

AGE

(V)

4.5

3.5

2.5

1.5

4.0

3.0

2.0

1.0

0.5

050 75–25

2873 G06

100250

RL = 100Ω

RL = 54Ω

RL = 100Ω

RL = 54Ω

VCC = 5VVCC = 3.3V

TEMPERATURE (°C)–50

DELA

Y (n

s)

50

40

30

20

10

050 75–25

2873 G07

100250

VCC = 3.3V, VL = 1.7VVCC = 5V, VL = 1.7VVCC = 3.3V, VL = 3.3VVCC = 5V, VL = 5V

TEMPERATURE (°C)–50 –25 0 25 50 75 100

–1.0

–0.5

0

0.5

1.0

1.5

2.0

SKEW

(ns)

2873 G08SHORT-CIRCUIT VOLTAGE (V)

–10

SHOR

T-CI

RCUI

T CU

RREN

T (m

A)

150

100

0

50

–50

–100

–15010–5

2873 G09

1550

OUTPUT LOW

OUTPUT HIGH

VCC = 5VVCC = 3.3V


DELA

Y (n

s)

80

70

60

50

4050 75–25

2873 G10

100250

VCC = 3.3V, VL = 1.7VVCC = 5V, VL = 1.7VVCC = 3.3V, VL = 3.3VVCC = 5V, VL = 5V

TEMPERATURE (°C)–50 –25 0 25 50 75 100

–1.0

–0.5

0

0.5

1.0

1.5

2.0

SKEW

(ns)

2873 G11OUTPUT CURRENT (mA)

0

OUTP

UT V

OLTA

GE (V

)

6

5

4

2

3

1

082

2873 G12

1064

VL = 5VVL = 3.3VVL = 1.7V


LTC2873

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RS232 Receiver Input Threshold vs Temperature

RS232 Receiver Output Voltage vs Load Current

RS485 Termination Resistance vs Temperature

RS232 Driver Switching at 250kbpsRS232 Operation at 1Mbps FAST Mode (DE485/F232 High) LTC2873 Drivers Changing Modes

RS232/RS485 Mode SwitchingTransition from Shutdown to RS232 Driver Output Going High and Low VDD and VEE Ripple


THRE

SHOL

D VO

LTAG

E (V

)

2.0

1.8

1.6

1.4

1.2

1.050 75–25

2873 G13

100250

VCC = 5VVCC = 3.3V

INPUT HIGH

INPUT LOW

OUTPUT CURRENT (mA)0

OUTP

UT V

OLTA

GE (V

)

6

5

4

2

3

1

082

2873 G14

1064

VL = 5VVL = 3.3VVL = 1.7V


RESI

STAN

CE (Ω

)

130

118

116

114

112

110

128

126

124

122

120

50 75–25

2873 G15

100250

VCM = –7VVCM = 2VVCM = 12V

FAST MODE SLOW MODE

2µs/DIV

DI3V/DIV

A3V/DIV

2873 G16400ns/DIV

RO3V/DIV

DI3V/DIV

A = B3V/DIV

2873 G17

TERMINATION ENABLED50pF FROM A/DO to B/RI,

20ns/DIV

DI5V/DIV

RO5V/DIV

A/DO1V/DIV

B/RI1V/DIV

2873 G18

RS232

RS485

2µs/DIV

DI

485/2325V/DIV

A/DO3V/DIV

B/RI

2873 G19

A HIGH

A LOW

SHDN

VEE

VDD

40µs/DIV

3V/DIV

2873 G20

10mV/DIV

2873 G2140µs/DIVRS485 READY MODE,ALL DRIVERS AND RECEIVERS DISABLED

VDD RIPPLE

VEE RIPPLE


LTC2873

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pin FuncTionsVEE (Pins 1, 10, 25): Generated Negative Supply Voltage for RS232 Driver (–6.3V). Tie all pins together and connect 1µF capacitor between VEE (Pin 10) and GND. Exposed pad (Pin 25) must be soldered to PCB to maintain low thermal resistance.

RO (Pin 2): RS485 Differential Receiver Output and RS232 Receiver Output. Logic level referenced to GND and VL.

485/232 (Pin3): Interface Select Input. A logic low enables RS232 mode and a high enables RS485 mode. The mode determines which transceiver inputs and outputs are ac-cessible at the LTC2873 pins. Logic level referenced to GND and VL. Do not float.

RE485 (Pin4): RS485 Receiver Enable. In RS485 mode, a logic high disables the RS485 receiver, leaving its output Hi-Z and a logic low enables the RS485 receiver. This input has no function in RS232 mode (485/232 low). Logic level referenced to GND and VL. Do not float.

DE485/F232 (Pin 5): RS485 Driver Enable and RS232 Fast Mode Enable. In RS485 mode (485/232 high), a logic low disables the RS485 driver leaving the driver outputs in a Hi-Z state and a logic high enables the RS485 driver. In RS232 mode (485/232 low), a logic high enables Fast mode with maximum data rate of 1Mbps. A logic low enables Slow mode with a maximum data rate of 250kbps. Logic level referenced to GND and VL. Do not float.

DI (Pin 6): RS485 and RS232 Driver Input. Logic level referenced to GND and VL. Do not float.

SHDN (Pin7): Shutdown Control. A logic low disables the LTC2873 into low power shutdown state, independent of the other inputs. Driver and receiver outputs become Hi-Z. Logic level referenced to GND and VL. Do not float.

GND (Pin 8, 11, 16, 20): Ground. Tie all four pins together.

CAP (Pin 9): Charge Pump Capacitor for Generated Negative Supply Voltage VEE. Connect a 220nF capacitor between CAP and SW.

SW (Pin 12): Switch Pin. Connect 10µH inductor between SW and VCC. See Inductor Selection section for further details.

VDD (Pin 13): Generated Positive Supply Voltage for RS232 Driver (+7.0V). Connect 1µF capacitor between VDD and GND.

VCC (Pin 14, 18, 21): Input Supply (3V to 5.5V). Tie all three pins together and connect a 2.2µF or larger capacitor between VCC (adjacent to VDD) and GND.

B/RI (Pin 15): RS485 Negative Receiver Input and Driver Output. In RS232 mode, this is the RS232 receiver input.

A/DO (Pin 17): RS485 Positive Receiver Input and Driver Output. In RS232 mode, this is the RS232 driver output.

NC (Pin 19): Not connected internally.

VL (Pin 22): Logic Supply (1.7V to 5.5V) for the Receiver Outputs, Driver Inputs, and Control Inputs. Bypass this pin to GND with a 0.1µF capacitor if not tied to VCC. Keep VL ≤ VCC for operation guaranteed to meet specifications. However, VL > VCC will not damage the device, provided that absolute maximum limits are respected. See “VL Logic Supply and Logic Pins” in Applications section for more information.

LB (Pin 23): Loopback Enable. A logic high enables logic loopback diagnostic mode, internally routing the driver input logic signals to the receiver output pins. This applies to RS232 and RS485 operation. The targeted receiver must be enabled for the loopback signal to be available on its output. A logic low disables Loopback mode. In Loopback mode, signals are not inverted from driver inputs to receiver outputs. Logic level referenced to GND and VL. Do not float.

TE485 (Pin24): RS485 Termination Enable. In RS485 mode, a logic low enables a 120Ω resistor between pins A/DO and B/RI. A logic high opens the resistor between A/DO and B/RI, leaving the pins unterminated. In RS485 mode, the 5k resistor between B/RI and GND is never engaged. In RS232 mode, the 120Ω resistor between A/DO and B/RI is never engaged, regardless of the state of TE485, and the 5k resistor between B/RI and GND is always engaged. Logic level referenced to GND and VL. Do not float.


LTC2873

102873fa


block DiagraM

2873 BD

CONTROLLOGIC

DRIVERS

RECEIVERSLOOPBACK

PATH

0.1µF

SHDN

485/232

DE485/F232

RE485

TE485

LB

DI

RO

GND

B/RI

A/DO

VEE

VDD

1.7V TO 5.5V(≤ VCC)

PULSE-SKIPPINGBOOST

REGULATORf = 1.2MHz

RT232

RT485

2.2µF

10µH 220nF3V TO 5.5V

1µF

1µF

CAPSWVCCVL

232

485

232

485

5k

RT232

125k

RT485

120Ω

125k


LTC2873

112873fa


Figure 1. RS485 Driver DC Characteristics Figure 2. RS485 Driver Output Current

Figure 3. RS485 Receiver Input Current and Resistance

Figure 4. RS485 Driver Timing Measurement

TesT circuiTs

2873 F01

DRIVERDIGND

ORVL

A/DORL

RLB/RI

VOD

+

–VOC

+

–

2873 F02

DRIVER

A/DO OR B/RI

DIGND

ORVL B/RI

OR A/DOVOUT

IOSD485

+–

2873 F03

RECEIVER

VINIIN485

RIN485 =

VIN

IIN485

+–

A/DO OR B/RI

B/RI OR A/DO

2873 F04

DRIVERDI

A/DO

B/RI

RDIFF

CL

CL

tPLHD485

tSKEWD485

tPHLD485

tRD485 tFD485

90%0V

VOD ½VOD

VL

0V

DI

B/RI

A/DO

A/RO – B/DI10%

90%0V

10%


LTC2873

122873fa


Figure 5. RS485 Driver Enable and Disable Timing Measurements

Figure 6. RS485 Receiver Propagation Delay Measurements

Figure 7. RS485 Receiver Enable and Disable Timing Measurements

TesT circuiTs

Figure 8. RS485 Termination Resistance and Timing Measurements

2873 F05

tZLD485,tZLSD485 tLZD485,

tLZSD485

tHZD485,tHZSD485

tZHD485,tZHSD485

½VCC

½VCC

VL

VOL

VCC

VOH

0V

0V

A/DO OR B/RI

B/RI OR A/DO

DRIVERDI

DE485/F232 OR SHDN

VLOR

GND

A/DO

RL

B/RI

GNDORVCC

VCCOR

GND

RL

CL

CL

½VL ½VLDE485/F232 OR SHDN

0.5V

0.5V

2873 F06

RECEIVERVCM

±VAB/2 A/DO

B/RI

RO

±VAB/2CL

tPLHR485

tSKEWR485 = tPLHR485 – tPHLR485

tRR485

90%

0V

½VL

A/DO TO B/RI

RO10%

tPHLR485

tFR485

90%

0V

–VAB

VAB

½VL 10%

VL

2873 F08

RECEIVER VAB

A/DO

B/RI

VAB

IARTERM =

VB

½VL ½VLTE485

IA

IA

90%10%

tRTZ485tRTEN4850V

VL

TE485

+–

+–

2873 F07

tZLR485,tZLRSR485

tLZR485,tLZSR485

tHZR485,tHZSR485

tZHR485,tZHRSR485

½VL

½VL

VL

VOL

VL

VOH

0V

0V

RE485 OR SHDN

RO

RO

RECEIVERRO

RE485 OR SHDN

0V TO 3V

3V TO 0V

A/DORL

B/RI

VLOR

GNDCL

½VL ½VL

0.5V

0.5V


LTC2873

132873fa


Figure 9. RS232 Driver Timing and Slew Rate Measurements

Figure 10. RS232 Driver Enable and Disable Times

TesT circuiTs

2873 F09

DI A/DO

CLRL

tPHLD232

tPLHD232

tSKEWD232 = |tPHLD232 – tPLHD232|

tF tR

DI

A/DO

SLEW RATE = 6V

tF OR tR

3V

–3V

0V

VOLD

VL

½VL ½VL

0V3V

–3V0V

VOHD

2873 F10

0V OR VL

SHDN

A/DO

CLRLtHZSD232

tLZSD232

SHDN

A/DO

A/DO

tZHSD232

tZLSD232

5V

–5V

0V

0V

0V

VOHD

VL

½VL ½VL

VOLD0.5V

0.5V

Figure 11. RS232 Receiver Timing Measurements

Figure 12. RS232 Receiver Enable and Disable Times

2873 F11

tPHLR232

tSKEWR232 = |tPLHR232 – tPHLR232|

tPLHR232

tRR232

90%

1.5V 1.5V

½VL 10%tFR232

90%

–3V

VL

+3V

½VL10% 0V

RO

RO

B/RI

B/RI

CL

2873 F12

–3V OR +3V

SHDN

RO GND

OR VL

CL

RL

tHZR232,tHZSR232

tLZR232,tLZSR232

SHDN

RO

RO

tZHR232,tZHSR232

tZLR232,

½VL

½VL

0V

0V

VL

VOHR

VL

½VL ½VL

VOLR

tZLSR232

0.5V

0.5V


LTC2873

142873fa


TesT circuiTs

SWVCC

C1220nFL1

10µH

LTC2873

VLDIRO

C42.2µF

2873 F13

SHDN485/232DE485/F232RE485TE485LB

VDD

VEE

VCC

CAP

B/RIA/DO

GND

C21µF

C31µF

+–V

tVEE

tVDD

SHDN

VDD

VEE

+6.3V

–5.9V

0V

½VL

0V

tRDY = MAX (tVDD, tVEE)

STEP

SWVCC

C1220nFL1

10µH

LTC2873

VLDIRO

C42.2µF

2873 F14

SHDN485/232DE485/F232RE485TE485LB

VDD

VEE

VCC

CAP

B/RIA/DO

GND

C41µF

C31µF

CB50pF

CA50pF

RA3k

+–V

tDR485

3V

0V

½VL ½VL

STEP

tR232

tR232

½VL ½VLRO

B

A

485/232

SWVCC

C1220nFL1

10µH

LTC2873

VLDIRO

C42.2µF

2873 F15

SHDN485/232DE485/F232RE485TE485LB

VDD

VEE

VCC

CAP

B/RIA/DO

GND

C21µF

C31µF

SWVCC

C1220nFL1

10µH

LTC2873

VLDIRO

C42.2µF

SHDN485/232DE485/F232RE485TE485LB

VDD

VEE

VCC

CAP

B/RIA/DO

GND

C21µF

C31µF

(a) (b)

HIGH FOR FAST MODELOW FOR SLOW MODE

Figure 13. Timing Coming Out of Shutdown Mode

Figure 14. Mode Change Timing

Figure 15. Testing Max Data Rate for (a) RS485 and (b) RS232. Observe that Data In Matches Data Out


LTC2873

152873fa


FuncTion TableKEY: 0 = Logic Low; 1 = Logic High; RX = Receiver; TX = Driver; l = Enabled; LB = Receiver Output is the Data Input Signal (Looped Back)

INPUTS RESULT

SHDN 485/232 RE485 DE485/F232 TE485 LB MODE DC/DC

CONV.RS232 RS485

RX TX RX TX TERM

0 X X X X X SHUTDOWN

1 0 X 0 X 0RS232 SLOW

l l l

1 0 X 0 X 1 l LB l

1 0 X 1 X 0 RS232 FAST

l l l

1 0 X 1 X 1 l LB l

1 1 1 0 1 X RS485 READY

l

1 1 1 0 0 X l l

1 1 0 0 0 0

RS485

l l l

1 1 0 0 0 1 l LB l

1 1 0 0 1 0 l l

1 1 0 0 1 1 l LB

1 1 0 1 0 0 l l l l

1 1 0 1 0 1 l LB l l

1 1 0 1 1 0 l l l

1 1 0 1 1 1 l LB l

1 1 1 1 0 X l l l

1 1 1 1 1 X l l


LTC2873

162873fa


Figure 16. Simplified DC/DC Converter with Required External Components

The LTC2873 is a flexible multiprotocol transceiver sup-porting RS485/RS422 and RS232 protocols.

This device can be powered from a single 3V to 5.5V supply with optional logic interface supply as low as 1.7V. An inte-grated DC/DC converter provides the positive and negative supply rails needed for RS232 operation. Automatically selected integrated termination resistors for both RS232 and RS485 protocols are included, eliminating the need for external termination components and switching relays. A logic loopback control is included for self-test and debug.

The LTC2873 bus interface is a single two-pin port that can be configured as either an RS232 driver/receiver pair or a differential RS485 (and RS422) transceiver depending on the state of the 485/232 pin. In RS485 mode, the driver and receiver can be enabled independently with the DE485/F232 and RE485 pins, or by tying these signals together, a single control selects transmit or receive modes. A 120Ω termination resistor is automatically engaged between pins A/DO and B/RI in RS485 mode if TE485 is low.

When the LTC2873 is in RS232 mode, the RS232 driver and receivers are both active and a 5k resistor is engaged at the receiver input to ground. The slew rate in RS232 mode can be set to support 1Mbps or 250kbps operation using the DE485/F232 pin.

applicaTions inForMaTionThe LTC2873 features rugged operation with ESD ratings of ±26kV HBM on the RS232 and RS485 receiver inputs and driver outputs, both unpowered and powered. All other pins offer protection exceeding ±4kV.

DC/DC Converter

The on-chip DC/DC converter operates from the VCC input, generating a 7.0V VDD supply and a charge pumped –6.3V VEE supply, as shown in Figure 16. VDD and VEE power the output stage of the RS232 drivers and are regulated to levels that guarantee greater than ±5V output swing.

The DC/DC converter requires a 10µH inductor (L1) and a bypass capacitor (C4) of 2.2µF. The charge pump capacitor (C1) is 220nF and the storage capacitors (C2 and C3) are 1µF. Locate C1 – C4 close to their associated pins shown in Figure 16. Refer to Layout Considerations section for guidance on circuit board layout.

Bypass capacitor C5 on the logic supply pin can be omitted if VL is connected to VCC. See the VL Logic Supply section for more details about the VL logic supply.

2873 F16

BOOSTREGULATOR

VCC3V TO 5.5V

VL1.7V TO VCC

C1220nF

L110µH

C42.2µF

VCC

VDD

VEE

SW

GNDGND

CAP

C50.1µF

C21µF

C31µF

VL

14 12 9

22

20

13

10

11

NOTE: NOT ALL PINS SHOWN. IN THE CASE OF DUPLICATE PINS FOR VCC, GND, AND VEE, EXTERNAL COMPONENTS SHOULD BE POSITIONED CLOSEST TO THE NUMBERED PIN SHOWN ABOVE.


LTC2873

172873fa


applicaTions inForMaTionPowering Multiple Devices

Multiple LTC2873 devices can be powered using the boost regulator from only one of the devices, requiring only one inductor (L1) and charge pump cap (C1). Since the RS232 drivers provide the primary load to the circuit, the following guidelines apply:

1. No more than four RS232 drivers can be supplied from a single device.

2. If more than two RS232 drivers are being supplied from a single device, then the inductor, L1, must be increased to 22µH and the charge pump cap, C1, must be increased to 470nH, and VDD and VEE bypass caps must be increased to 2.2µF.

3. Ground the SW pin on devices with inactive boost converters.

4. Connect CAP pins together for all devices.

5. Connect VEE pins together for all devices.

6. Connect VDD pins together for all devices.

Figures 32 shows an example of how to connect four devices.

Inductor Selection

A 10µH or 22μH (±20%) inductor with a saturation current (ISAT) rating of at least 220mA and a DCR (copper wire resistance) of less than 1.3Ω is required. Some very small inductors meeting these requirements are listed in Table 1.

Capacitor Selection

The small size of ceramic capacitors makes them ideal for the LTC2873. Use X5R or X7R dielectric types; their

ESR is low and they retain their capacitance over relatively wide voltage and temperature ranges. Use a voltage rating of at least 10V.

Running with External VDD and VEE Supplies

The inductor and charge pump cap, C1, can be omitted only if VDD and VEE are externally supplied. Bypass caps on VDD and VEE must remain in place. In this circumstance, ground the SW pin and float the CAP pin. External supplies must not exceed the absolute maximum levels of ±7.5V. Ideal supply levels are 7.2V and –6.5V as these are each just wider than the regulation points of 7.0V and –6.3V so the internal feedback is satisfied and the switching stops. Lower voltages can be used even at –6V and +6V but the internal boost regulator will be switching. This may cause some switching noise but will not harm the part. VDD and VEE supplies must be present for proper operation in RS232 mode and in RS485 mode when the termination is enabled. It is okay to run the LTC2873 in RS485 mode with internal termination disabled (TE485 high), when VDD and VEE are not present or fully settled.

Inrush Current and Supply Overshoot Precaution

In certain applications, fast supply slew rates are generated when power is connected. If the VCC voltage is greater than 4.5V and its rise time is faster than 10µs, the pins VDD and SW can exceed their absolute maximum values during start-up. When supply voltage is applied to VCC, the voltage difference between VCC and VDD generates inrush current flowing through inductor L1 and capacitors C1 and C2. The peak inrush current must not exceed 2A. To avoid this condition, add a 1Ω resistor as shown in Figure 17. This precaution is not relevant for supply voltages below 4.5V or rise times longer than 10µs.

Table 1. Recommended Inductors

PART NUMBER L (μH) ISAT (mA) MAX DCR (Ω) SIZE (mm) MANUFACTURER

74479888310 10 250 0.5 2.5 × 2 × 1 Wurth Elektronik

CBC2016T100K (or M) 10 380 1.07 2 × 1.6 × 1.6Taiyo Yuden www.t-yuden.comCBC2518T220K (or M) 22 320 1.0 2.5 × 1.8 × 1.8

BRC2016T220K (or M) 22 310 1.3 2 × 1.6 × 1.6

LQH32CN220K53 22 250 0.92 3.2 × 2.5 × 1.6 Murata www.murata.com


LTC2873

182873fa


applicaTions inForMaTion

Figure 17. Supply Current Overshoot Protection for Input Supplies of 4.5V or Higher and Rise Times Faster Than 10μs

VL Logic Supply and Logic Pins

A separate logic supply pin VL allows the LTC2873 to interface with any logic signal from 1.7V to 5.5V. All logic I/Os use VL as their high supply. For proper operation, VL should not be greater than VCC. During power-up, if VL is higher than VCC, the device will not be damaged, but behavior of the device is not guaranteed. In particular, supply currents can be somewhat higher than specified. If VL is not connected to VCC, bypass VL with a 0.1µF capacitor to GND.

RS232 and RS485 driver outputs are undriven and the RS485 termination resistors are disabled when VL or VCC is grounded or VCC is disconnected.

Although all logic input pins reference VL as their high supply, they can be driven up to 7V, independent of VL and VCC, with the exception of SHDN, which must not exceed VL by more than 0.3V. Logic input pins do not have internal biasing devices to pull them up or down. They must be driven high or low to establish valid logic levels; do not float.

RS485 Driver

The RS485 driver provides full RS485/RS422 compatibility. When enabled, if DI is high, (A/DO to B/RI) is positive. With the driver disabled, the A/DO and B/RI resistance is

2873 F17

0V

5V

≤10µs

C1220nF

L110µH

INRUSHCURRENTC4

2.2µF

R11Ω1/8W

VCC

VDD GND

SW CAP

C21µF

14

12 9

13 11

greater than 96kΩ (typically 125kΩ) to ground over the entire common mode range of –7V to +12V. This resistance is actually the RS485 receiver input resistance, which is connected to the same pins.

RS232 Driver with Speed Selection

The RS232 driver provides full compatibility with the TIA/EIA-232-F (RS232) specification. When in RS232 mode, the driver is automatically enabled. Like all RS232 driv-ers, it is inverting, so that when the input, DI, is low, the output, A/DO, is high, and vice-versa.

The RS232 driver slew rate can be selected to support data rates of up to 250kbps or 1Mbps with the DE485/F232 pin. Since RS232 signals are single ended and large amplitude, compared with RS485, radiated emissions may be a concern. To minimize emissions, the speed selection should be set to Slow mode by setting DE485/F232 low for data rates of 250kbps or less. For higher data rates, up to 1Mbps, Fast mode must be engaged by setting DE485/F232 high. Even in Fast mode the driver transitions are slew controlled to minimize emissions. See Typical Performance Characteristics section for examples of the waveforms.

Driver Overvoltage and Overcurrent Protection

The RS232 and RS485 driver outputs are protected from short circuits to any voltage within the absolute maximum range of ±15V. The maximum current in this condition is 90mA for the RS232 driver and 250mA for the RS485 driver. If the RS485 driver output is shorted to a voltage greater than VCC, when it is active, positive current of about 100mA can flow from the driver output back to VCC. If the system power supply or loading cannot sink this excess current, clamp VCC to GND with a Zener diode (e.g., 5.6V, 1W, 1N4734) to prevent an overvoltage condition on VCC.

All devices also feature thermal shutdown protection that disables the drivers, receivers, and RS485 terminators in case of excessive power dissipation during momentary overload conditions. Overtemperature protection activates at a junction temperature exceeding about 165°C (not tested in production). NOTE: Continuous operation above the specified maximum operating junction temperature may result in device degradation or failure.


LTC2873

192873fa


Figure 18. RS485 Receiver Input Threshold Characteristics with Typical Values Shown

2873 F18

–110mV –70mV 0V

RA

+110mVA/DO TO B/RI

RISING THRESHOLDSHIFTS IF SIGNAL ISIN WINDOW > ~1.3µs

TO SUPPORTFAILSAFE

Typical applicaTionsRS485 Balanced Receiver with Full Failsafe Operation

The LTC2873 RS485 receiver has a differential threshold voltage that is about +110mV for signals that are rising and –110mV for signals that are falling, as illustrated in Figure 18. If a differential input signal lingers in the window between these thresholds for more than about 1.3µs, the rising threshold changes from +110mV to –70mV, while the falling threshold remains at –110mV. Thus, differential inputs that are shorted, open, or terminated but not driven for more than 1.3µs produce a high on the receiver output, indicating a failsafe condition.

(A-B)200mV/DIV

B200mV/DIV

A

RO5V/DIV

2873 F19200ns/DIV

Figure 19. A 3Mbps Signal Driven Down 4000ft of CAT-5e Cable. Top Traces: Received Signals After Transmission Through Cable; Middle Trace: Math Showing Differences of Top Two Signals; Bottom Trace: Receiver Output

The benefit of this dual threshold architecture is that it supports full failsafe operation yet offers a balanced threshold, centered on 0V, for normal data signals. This balance preserves duty cycle for small input signals with heavily slewed edges, typical of what might be seen at the end of a very long cable. This performance is highlighted in Figure 19, where a signal is driven through 4000ft of CAT-5e cable at 3Mbps. Even though the differential sig-nal peaks are at only 200mV and is heavily slewed, the output maintains a nearly perfect signal with almost no duty cycle distortion.

An additional benefit of the balanced architecture is excel-lent noise immunity due to the wide effective differential input signal hysteresis of 220mV for signals transitioning through the window region in less than 1.3µs. Increasingly slower signals will have increasingly less effective hyster-esis, limited by the DC failsafe hysteresis of about 40mV.

RS485 Biasing Network Not Required

RS485 networks are often biased with a resistive divider to generate a differential voltage of ≥200mV on the data lines, which establishes a logic-high state when all the transmitters on the network are disabled. The values of the biasing resistors depend on the number and type of transceivers on the line and the number and value of terminating resistors. Therefore, the values of the biasing resistors must be customized to each specific network installation, and may change if nodes are added to or removed from the network.

The internal failsafe feature of the LTC2873 eliminates the need for external network biasing resistors provided they are used in a network of transceivers with similar internal failsafe features. This also allows the network to support a high number of nodes, up to 256, by eliminating the bias resistor loading. The LTC2873 transceiver operates correctly on biased, unbiased, or under-biased networks.

If a twisted pair has unbalanced capacitance from its two conductors to AC ground, common mode transients can translate into small differential voltages. If the common mode event is large and fast enough, the resulting dif-ferential voltage can cause a receiver, whose inputs are


LTC2873

202873fa


Figure 20. Equivalent RS485 Receiver Input Resistance Into A/DO and B/RI

2873 F20

A/DO

B/RI

TE485

60Ω

60Ω

125k

125k

undriven, to change state momentarily. In these extreme conditions, high quality shielded cable is recommended. If necessary, biasing resistors can be used on the bus to pull the resting signal farther from the receivers failsafe threshold.

Receiver Outputs

The RS232 and RS485 receiver outputs are internally driven high (to VL) or low (to GND) with no external pull up needed. When the receivers are disabled the output pin becomes Hi-Z with leakage of less than ±5µA for voltages within the VL supply range.

RS485 Receiver Input Resistance

In RS485 mode, the RS485 receiver input resistance from A/DO or B/RI to GND is 125kΩ (typical) when the inte-grated termination is disabled. This permits up to a total of 256 receivers per system without exceeding the RS485 receiver loading specification. The input resistance of the receiver is unaffected by enabling/disabling the receiver or whether the part is in loopback mode, or unpowered. The equivalent input resistance looking into the RS485 receiver pins is shown in Figure 20.

Figure 21. Typical Resistance of the Enabled RS485 Terminator vs Common Mode Voltage of A/DO and B/RI

VOLTAGE (V)–10

RESI

STAN

CE (Ω

)

126

124

122

118

120

11610–5

2873 F21

1550

VCC = 5.0VVCC = 3.3V

applicaTions inForMaTionSelectable RS485 Termination

Proper cable termination is important for good signal fidel-ity. When the cable is not terminated with its characteristic impedance, reflections cause waveform distortion.

The LTC2873 offers an integrated switchable 120Ω ter-mination resistor between pins A/DO and B/RI.

This termination supports communication over a twisted pair cable with characteristic impedance of 120Ω or 100Ω, including CAT-5 cables. It has the advantage of being able to easily change, through logic control, the proper line termination for correct operation when configuring transceiver networks. Termination should be enabled on transceivers positioned at both ends of the network bus only. However, the driving end of a line does not need to be terminated. By turning off termination at the driver, the reduced load results in less power dissipation and a larger signal swing on the bus. TE485 can be tied to DE485/F232 to logically switch the termination on only when the driver is inactive if the termination enable/disable delays can be tolerated in the overall system level timing. If the delays are not acceptable, tie TE485 low to enable termination for all modes of RS485 operation, whether driving or receiving.

The termination resistance is maintained over the entire RS485 common mode range of –7V to 12V as shown in Figure 21. The voltage across pins with the terminating resistor enabled should not exceed 6V as indicated in the Absolute Maximum Ratings table.

RS232 Receiver Input Resistance

In RS232 mode, the receiver input resistance on the B/RI pin is always 5k to GND. In any other mode, this resistor is switched out. The 120Ω RS485 termination resistor between pins A/DO and B/RI is never engaged in RS232 mode, regardless of the state of TE485 pin.


LTC2873

212873fa


applicaTions inForMaTionLogic Loopback

A loopback mode connects the driver inputs to the receiver outputs (non-inverting) providing an echo for self-test. This applies to both RS232 and RS485 transceivers. Loopback mode is entered when the LB pin is set to a logic-high and the relevant receiver is enabled. The RS485 driver output can be disabled in loopback mode if DE485/F232 is held low, or functions normally with DE485/F232 high. The RS232 driver output cannot be disabled when loopback is engaged in RS232 mode, and functions normally. The loopback signal traverses a path from the logic input cir-cuit at DI to the logic output at RO and does not exercise the entire driver or receiver circuit. Thus loopback, alone, is not a sufficient test to ensure full functionality of the LTC2873. Loopback does not affect the operation of the termination resistors.

Robust ESD Protection

The LTC2873 features exceptionally robust ESD protection. The transceiver interface pins (A and B) are protected to ±26kV human body model with respect to GND, VCC, or VL without latchup or damage. This protection holds whether the device is unpowered or powered in any mode of opera-tion. To note, ±26kV is an upper limit of the tester—the actual device protection level is higher. Every other pin on the device is protected to ±4kV ESD (HBM) for all-around robustness. Figure 22 shows the LTC2873 being struck repeatedly with 26kV of ESD energy (air gap discharge) during operation with no damage or circuit latchup.

RS485 Cable Length vs Data Rate

Many factors contribute to the maximum cable length that can be used for RS485 or RS422 communication, including driver transition times, receiver threshold, duty

Figure 22. LTC2873 Struck Repeatedly with 26kV of ESD Energy While Operating. No Damage or Circuit Latchup Occurs


LTC2873

222873fa


Figure 23. Cable Length vs Data Rate (RS485/RS422 Standard Shown in Vertical Solid Line)

DATA RATE (bps)

CABL

E LE

NGTH

(FT)

2873 F23

10k

1k

100

1010k 10M 100M1M100k

LTC2873MAX DATA RATE

RS485/RS422MAX DATA RATE

cycle distortion, cable properties and data rate. A typical curve of cable length versus maximum data rate is shown in Figure 23. Various regions of this curve reflect different performance limiting factors in data transmission.

At frequencies below 100kbps, the maximum cable length is determined by DC resistance in the cable. In this ex-ample, a cable longer than 4000ft will attenuate the signal at the far end to less than what can be reliably detected by the receiver.

For data rates above 100kbps, the capacitive and inductive properties of the cable begin to dominate this relation-ship. The attenuation of the cable is frequency and length dependent, resulting in increased rise and fall times at the far end of the cable. At high data rates or long cable lengths, these transition times become a significant part of the signal bit time. Jitter and inter symbol interference aggravate this so that the time window for capturing valid data at the receiver becomes impossibly small.

The boundary at 20Mbps in Figure 23 represents the guaranteed maximum operating rate of the LTC2873. The dashed vertical line at 10Mbps represents the specified maximum data rate in the RS485 standard. This boundary is not a limit, but reflects the maximum data rate that the specification was written for. It should be emphasized that the plot in Figure 23 shows a typical relation between maximum data rate and cable length. Results with the LTC2873 will vary, depending on cable properties such as conductor gauge, characteristic impedance, insulation material, and solid versus stranded conductors.

applicaTions inForMaTionLayout Considerations

All VCC pins must be connected together and all ground pins must be connected together on the PC board with very low impedance traces or dedicated planes. A 2.2µF, or larger, bypass capacitor should be placed less than 7mm away from VCC Pin 14. This VCC pin, as well as GND Pin 11, mainly service the DC/DC converter. Additional bypass capacitors of 0.1µF or larger, can be added from VCC pin 18 to ground pin 16 if the traces back to the 2.2µF capacitor are indirect or narrow. These VCC and ground pins mainly service the RS485 driver. Table 2 summarizes the bypass capacitor requirements. The capacitors listed in the table should be placed closest to their respective supply and ground pin.

Table 2. Bypass Capacitor RequirementsCAPACITOR (µF) SUPPLY (PIN) RETURN (PIN) COMMENT

2.2 VCC (14) GND (11) Required

1.0 VDD (13) GND (11) Required

1.0 VEE (10) GND (11) Required

0.1 VL (22) GND (20) Required*

0.1 VCC (18) GND (16) Optional

0.1 VCC (21) GND (20) Optional

*If VL is not connected to VCC.

Place the charge pump capacitor, C1, directly adjacent to the SW and CAP pins, with no more than one centimeter of total trace length to maintain low inductance. Close placement of the inductor, L1, is of secondary importance compared to the placement of C1 but should include no more than two centimeters of total trace length.

The PC board traces connected to high speed bus signals A/DO and B/RI should be symmetrical and as short as possible to minimize capacitive imbalance and to maintain good differential signal integrity. To minimize capacitive loading effects, the differential signals should be separated by more than the width of a trace and should not be routed on top of each other if they are on different signal planes.

Care should be taken to route outputs away from any sensitive inputs to reduce feedback effects that might cause noise, jitter, or even oscillations. For example, DI, A/DO, and B/RI should not be routed near RO.


LTC2873

232873fa


Typical applicaTions Supply Connections and External Components Necessary for Operation Are Not Shown. H = Logic High; L = Logic Low, X = Don’t Care (Logic High or Logic Low)

Figure 25. RS485 Configurations

2873 F25

LTC2873

RS485 TX + RX + TERM

A/DO

B/RI

DI

RO

SHDN

485/

232

RE48

5

TE48

5

DE48

5/F2

32

LB

H H H L L L

LTC2873

RS485 TX + TERM

A/DO

B/RI

DI

RO

SHDN

485/

232

RE48

5

TE48

5

DE48

5/F2

32

LB

H H H H L L

LTC2873

RS485 RX + TERM

A/DO

B/RI

DI

RO

SHDN

485/

232

RE48

5

TE48

5

DE48

5/F2

32

LB

H H L L L L

LTC2873

RS485 TX + RX

A/DO

B/RI

DI

RO

SHDN

485/

232

RE48

5

TE48

5

DE48

5/F2

32

LB

H H H L H L

LTC2873

RS485 TX

A/DO

B/RI

DI

RO

SHDN

485/

232

RE48

5

TE48

5

DE48

5/F2

32

LB

H H H H H L

LTC2873

RS485 RX

A/DO

B/RI

DI

RO

SHDN

485/

232

RE48

5

TE48

5

DE48

5/F2

32

LB

H H H L H L

120Ω 120Ω120Ω

Figure 24. RS232 Configurations

2873 F24

5k

LTC2873

RS232 SLOW

A/DO

B/RI

DI

RO

SHDN

485/

232

RE48

5

TE48

5

DE48

5/F2

32

LB

H L L X X L

5k

LTC2873

RS232 FAST

A/DO

B/RI

DI

RO

SHDN

485/

232

RE48

5

TE48

5

DE48

5/F2

32

LB

H L H X X L

5k

LTC2873

RS232 + LOOPBACK

A/DO

B/RI

DI

RO

SHDN

485/

232

RE48

5

TE48

5

DE48

5/F2

32

LB

H L X X X H


LTC2873

242873fa


Figure 26. RS485 + Loopback Configurations

Figure 27. Shutdown, RS485 Ready and RS485 Ready + Term Configurations


2873 F26

LTC2873

RS485 LOOPBACK + TX + TERM

A/DO

B/RI

DI

RO

SHDN

485/

232

RE48

5

TE48

5

DE48

5/F2

32

LB

H H H L L H

120Ω

LTC2873

RS485 LOOPBACK + TX

A/DO

B/RI

DI

RO

SHDN

485/

232

RE48

5

TE48

5

DE48

5/F2

32

LB

H H H L H H

120Ω

LTC2873

RS485 LOOPBACK + TERM

SHDN

485/

232

RE48

5

TE48

5

DE48

5/F2

32

LB

H H H L L H

120Ω

LTC2873

RS485 LOOPBACK

A/DO

B/RI

DI

ROSH

DN

485/

232

RE48

5

TE48

5

DE48

5/F2

32

LB

H H L L H H

A/DO

B/RI

DI

RO

2873 F27

LTC2873

DC/DCCONVOFF

SHUTDOWN

A/DO

B/RI

DI

RO

SHDN

485/

232

RE48

5

TE48

5

DE48

5/F2

32

LB

L X X X X X

LTC2873

DC/DCCONV

ON

A/DO

B/RI

DI

RO

SHDN

485/

232

RE48

5

TE48

5

DE48

5/F2

32

LB

H H L H H L

RS485 READY

LTC2873

DC/DCCONV

ON

A/DO

B/RI

DI

RO

SHDN

485/

232

RE48

5

TE48

5

DE48

5/F2

32

LB

H H L H L L

RS485 READY + TERM

120Ω


LTC2873

252873fa


Figure 28. Typical RS485 Half Duplex Network

Figure 29. Typical RS232 Communications Link


2873 F28

LTC2873

DI

RO

A/DO

B/RI

SHDN

485/

232

RE48

5

TE48

5

DE48

5/F2

32

LB

H H L L

120Ω

LTC2873

A/DO

B/RI

DI

RO

SHDN

485/

232

RE48

5

TE48

5

DE48

5/F2

32

LB

H H L L

120Ω

LTC2873

A/DO

B/RI

DI

RO

SHDN

485/

232

RE48

5

TE48

5

DE48

5/F2

32

LB

H H H L

LTC2873

DI

RO

A/DO

B/RI

SHDN

485/

232

RE48

5

TE48

5

DE48

5/F2

32

LB

H H H L

D R

D RD R

D R

H L XX L

2873 F29

5k

LTC2873

A/DO

B/RI

DI

RO

SHDN

485/

232

RE48

5

TE48

5

DE48

5/F2

32

LB

H L XX L

5k

LTC2873

DI

RO

A/DO

B/RI

SHDN

485/

232

RE48

5

TE48

5

DE48

5/F2

32

LB

FAST SLOWFAST SLOW


LTC2873

262873fa


Figure 30. Low Voltage Microprocessor Interface

Figure 31. Receiver-Only RS485 Termination for Power Savings


2873 F30

3V TO 5.5V

1.7V TO VCC

µP

LTC2873

LOGICLEVEL

SIGNALS

LINELEVEL

SIGNALSRS232AND/ORRS485

VCC

VL

GND

2873 F31

120ΩDI

RO

485/232

RE485TE485

DE485/F232

A/DO

B/RI

LTC2873SH

DN

LB

H H

Y X MODE L H H RS485 RECEIVE TRANSMIT L RS232 SLOW FAST

Y

X


LTC2873

272873fa


Typical applicaTions

APPLICATIONNUMBER OF

LTC2873 DEVICES L1 C1 C2 C3MINIMUM

C4-X

SINGLE TRANSCEIVER 1 10µH 220nF 1µF 1µF 2.2µF

DUAL TRANSCEIVER 2 10µH 220nF 1µF 1µF 2.2µF

TRIPLE TRANSCEIVER 3 22µH 470nF 2.2µF 2.2µF 1µF

QUAD TRANSCEIVER 4 22µH 470nF 2.2µF 2.2µF 1µF

Figure 32. Quad Transceiver

DI-ARO-A

VCCVL

SHDN485/232

LBTE485

DIRO

DE485/F232RE485

SHDN485/232-A

LBTE485-A

DX485-ARX485-A

SWCAPVDDVEE

GND

A/DOB/RI

A/DO-AB/RI-A

L122µH

C1470nF

C4-A1µF LTC2873

2873 F32

LTC2873

LTC2873

LTC2873

A

DI-BRO-B

VCCVL

SHDN485/232

LBTE485

DIRO

DE485/F232RE485

485/232-B

TE485-B

DX485-BRX485-B

SWCAPVDDVEE

GND

A/DOB/RI

A/DO-BB/RI-B

B

C4-B1µF

DI-CRO-C

VCCVL

SHDN485/232

LBTE485

DIRO

DE485/F232RE485

485/232-C

TE485-C

DX485-CRX485-C

SWCAPVDDVEE

GND

A/DOB/RI

A/DO-CB/RI-C

C

C4-C1µF

DI-DRO-D

VCCVL

SHDN485/232

LBTE485

DIRO

DE485/F232RE485

485/232-D

TE485-D

DX485-DRX485-D

SWCAPVDDVEE

GND

A/DOB/RI

A/DO-DB/RI-D

D

C4-D1µF

C32.2µF

C22.2µF

3V TO 5.5V


LTC2873

282873fa


package DescripTionPlease refer to http://www.linear.com/product/LTC2873#packaging/ for the most recent package drawings.

4.00 ±0.10(2 SIDES)

5.00 ±0.10(2 SIDES)

NOTE:1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WXXX-X).2. DRAWING NOT TO SCALE3. ALL DIMENSIONS ARE IN MILLIMETERS4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE5. EXPOSED PAD SHALL BE SOLDER PLATED6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE

PIN 1TOP MARK(NOTE 6)

0.40 ±0.10

23 24

1

2

BOTTOM VIEW—EXPOSED PAD

0.75 ±0.05R = 0.115TYP

R = 0.05 TYP PIN 1 NOTCHR = 0.20 OR C = 0.35

0.25 ±0.05

0.50 BSC

0.200 REF

0.00 – 0.05

(UFD24) QFN 0506 REV A

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONSAPPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED

0.70 ±0.05

0.25 ±0.050.50 BSC

2.65 ±0.052.00 REF

3.00 REF4.10 ±0.055.50 ±0.05

3.10 ±0.05

4.50 ±0.05

PACKAGE OUTLINE

2.65 ±0.10

2.00 REF

3.00 REF3.65 ±0.10

3.65 ±0.05

UFD Package24-Lead Plastic QFN (4mm × 5mm)

(Reference LTC DWG # 05-08-1696 Rev A)


http://www.linear.com/product/LTC2873#packaging/

LTC2873

292873fa


Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

revision hisToryREV DATE DESCRIPTION PAGE NUMBER

A 05/16 Applied Note 7 to tZLSR232, tZHSR232.Added Exposed Pad soldering requirement to VEE pin description.Corrected recommended Wurth inductor part number.

69

17


LTC2873

302873fa

For more information www.linear.com/LTC2873 LINEAR TECHNOLOGY CORPORATION 2016

LT 0516 REV A • PRINTED IN USALinear Technology Corporation1630 McCarthy Blvd., Milpitas, CA 95035-7417(408) 432-1900 FAX: (408) 434-0507 www.linear.com/LTC2873

relaTeD parTs

Typical applicaTion

PART NUMBER DESCRIPTION COMMENTS

LTC2870, LTC2871 RS232/RS485 Multiprotocol Transceiver with Integrated Termination

Two RS232 and One RS485 Transceivers. 3V to 5.5V Supply, Automatic Selection of Termination Resistors, Duplex Control, Logic Supply Pin, Up to ±26kV ESD

LTC2872 RS232/RS485 Dual Multiprotocol Transceiver with Integrated Termination

Four RS232 and Two RS485 Transceivers. 3V to 5.5V Supply, Automatic Selection of Termination Resistors, Duplex Control, Logic Supply Pin, ±15kV ESD

LTC1334 Single 5V RS232/RS485 Multiprotocol Transceiver Dual Port, Single 5V Supply, Configurable, ±10kV ESD

LTC1387 Single 5V RS232/RS485 Multiprotocol Transceiver Single Port, Configurable

LTC2801/LTC2802/ LTC2803/LTC2804

1.8V to 5.5V RS232 Single and Dual Transceivers Up to 1Mbps, ±10kV ESD, Logic Supply Pin, Tiny DFN Packages

LTC2854/LTC2855 3.3V 20Mbps RS485 Transceiver with Integrated Switchable Termination

3.3V Supply, Integrated, Switchable, 120Ω Termination Resistor, ±25kV ESD

LTC2859/LTC2861 20Mbps RS485 Transceiver with Integrated Switchable Termination

5V Supply, Integrated, Switchable, 120Ω Termination Resistor, ±15kV ESD

LTM®2881 Complete Isolated RS485/RS422 µModule® Transceiver + Power

20Mbps, 2500VRMS Isolation with Integrated DC/DC Converter, Integrated Switchable 120Ω Termination Resistor, ±15kV ESD

LTM2882 Dual Isolated RS232 µModule Transceiver + Power 1Mbps, 2500VRMS Isolation with Integrated DC/DC Converter, ±10kV ESD

3500VRMS Isolated RS485/RS232 Transceiver

2873 TA02

120Ω

5k

1μF

ISOLATED5V IN220nF 10μH

2.2μF

ISO

ISO

VLCAP SW

SHDN

DI

RO

485/232

RE485TE485

DE485/F232

LB

GND

RS232

RS485

A/DO

B/RI

VDD

VEE

VCC

1μF

ISOLTC2873

ISOLATEDGND IN

3V TO5.5V

DI

RO

485/232RS485 D R

ISO

10k

RS232(485/232

LOW)

RS485(485/232

HIGH)

VCC2VL2

ON2INDINEINF

OUTCOUTBOUTA

EOUTAGND2GND2

VCC1VL1ON1OUTDOUTEOUTFINCINBINAEOUTDGND1GND1

LTM2892-S

ISOL

ATIO

N BA

RRIE

R
















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ltc2873 - single-bus rs485/rs232 multiprotocol …€¦ · loopback mode steers the driver inputs...

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